Every protocol/technology is different. For example, the Astro, Weather, MQTT, and HTTP bindings just need networking. Zwave requires a USB dongle. GPIO obviously requires jumper wires connected to the GPIO pins on your computer (presumably a board computer like Raspberry Pi or BeagleBone or the like).
So if you have specific questions about a specific technology you are looking at I’m sure there will be a wealth of advice you can illicit from these forums. Similarly the wiki page for that technology’s binding will also contain warnings and hint for how to get it working.
In short, there is no one answer nor one set of recommendations. Furthermore, connecting the hardware to your server is often as simple as plugging in a USB dongle (e.g. zwave), or setting up a networking connection with another hub (e.g. phillips hue). There really isn’t anything special for setting up the hardware, unless you are going for something completely DIY like Arduino boards with relays and sensors, in which case you are asking for help with that on the wrong forum.
Not really one page. Like I said, each supported technology has its own wiki page and on that page will be a lot of info regarding required hardware and how to set it up.
Not that I’m aware of. For the most part OH is more of a DIY community as far as I can tell thus far. But I would be surprised if there are not companies using it, they just are not posting about it on these forums.